Trying to customize my main menu but no luck

Hello,
I’ struggling with understanding the concept of custom menus in WP. I have searched and read numerous tutorials, but most seem to be treating much more intricate and difficult topics than the one I’m having.
I have a CSS-class with some styling that I would like to apply to the main menu (and because I know I will use one more navigation later on, and it didn’t seem to add much difficulties, I register one extra menu as well in functions.php.

I’ve been reading through and tryied to apply the code from https://codex.www.ads-software.com/Function_Reference/register_nav_menu
https://wordpress.stackexchange.com/questions/32186/how-to-create-a-custom-wordpress-navigation-menu-with-the-code
and https://codex.www.ads-software.com/Navigation_Menus

But my WP doesn’t seem to care at all; my menu is still looking like an unstyled unordered bulleted list.

Here’s my code:
Functions.php:

function register_my_menus() {
            register_nav_menus(
                    array(
                        'header-menu' => __('Header Menu'),
                        'extra-menu' => __('Extra Menu')
                    )
                    );
}
        add_action('init', 'register_my_menus');

In Header.php:

<div id="navbar">

	<?php wp_nav_menu( array( 'theme_location' =>   'header-menu' , 'container_class' => 'bmenu' )); ?>

        </div><!--End div id navbar -->

And the corresponding css (an excerpt, it would be too long with all the different selectors):

.bmenu{
	styling...bla bla bla
}
.bmenu li{
	styling... and so on
}

Can anyone please help out? I really want to know where I have gone wrong.